The heartbeat of a mammal can be measured by determination of light absorption upon passage through the skin. For this purpose, for instance, on one side of the skin, light of a known intensity is generated, and on the opposite side the magnitude of the fraction of this light that is transmitted by the skin is measured. The measured fraction depends on inter alia the extent to which light en route is absorbed in the skin by the blood in the veins and capillaries in it. This, in turn, depends on inter alia the diameter of the veins and capillaries. This diameter varies periodically with the frequency of the heartbeat. Accordingly, the measured intensity also varies with that same frequency. This corresponding frequency of the intensity variations is then recorded and stored or forwarded for further processing.